The present invention relates to a method for the preparation of a rare earth-based permanent magnet having high corrosion resistance as well as to a rare earth-based permanent magnet having high corrosion resistance obtained by the method. More particularly, the invention relates to a method for imparting high corrosion resistance to a rare earth/iron/boron permanent magnet as well as to a rare earth/iron/boron permanent magnet having high corrosion resistance obtained by the method.
As is well known, rare earth-based permanent magnets in general have great advantages as compared with other types of non-rare earth permanent magnets in respects of their excellent magnetic properties and economical merits by virtue of remarkable compactness of the permanent magnets so that they are widely employed in the fields of electric and electronic instruments. Rare earth-based permanent magnets are now on a stage of further development where they are required to be of more and more improved magnetic performance in order to comply with the recent trend in the electric and electronic technologies.
Among several classes of rare earth-based permanent magnets heretofore developed, the so-called rare earth/iron/boron permanent magnets or, typically, neodymium/iron/boron permanent magnets are the most prominent as compared with the earlier developed samarium/cobalt permanent magnets in respects of the much superior magnetic properties and much lower material costs because neodymium is much more abundant as a rare earth resource than samarium and no or only a small amount of expensive cobalt is required in the formulation of the magnet alloy composition. Accordingly, neodymium/iron/boron permanent magnets are highlighted and expected in the near future to substitute not only for samarium/cobalt permanent magnets conventionally employed in a compact-size magnetic circuit but also for hard ferrite permanent magnets of a relatively large size and certain large electromagnets.
Rare earth/iron/boron permanent magnets in general, however, have a serious disadvantage that, as an inherence of the rare earth element or neodymium and iron as the principal metallic constituents of the magnet alloy composition, the magnet is readily oxidized on the surface within a short time when kept in an atmosphere of moisture-containing air. When oxidation takes place on the surface of a rare earth/iron/boron permanent magnet built in an electric or electronic instrument, a decrease is unavoidable in the performance of the magnetic circuit if not to mention the problem of contamination of ambience by the rust particles formed by oxidation and falling off the magnet surface.
With an object to improve corrosion resistance of a rare earth/iron/boron permanent magnet, proposals are made heretofore for methods to provide the magnet surface with a protective coating layer such as a resinous coating layer and a metallic coating layer of, for example, nickel which is formed by a dry-process vapor-phase deposition method, e.g., ion plating, or by a wet-process electrolytic plating method. These surface coating methods are practically not feasible due to the high costs requited for the process which is necessarily very complicated.
In view of the problem of high costs in the above mentioned surface coating methods, a simpler and less expensive surface treatment method is proposed in Japanese Patent Kokai 6-302420, according to which the surface treatment of a rare earth/iron/boron permanent magnet is finished by a chromic acid treatment alone. This method, however, cannot be very inexpensive by all means because the chromic acid treatment must be preceded by a pickling treatment with an acid such as nitric acid and the spent chromic acid solution, which is notoriously toxic to cause heavy environmental pollution, must be disposed with complete safety necessarily requiring a high cost.
As an alternative of the above mentioned chromic acid treatment having problems relative to the high costs and difficulty in the waste disposal, a method is proposed in Japanese Patent Kokai 9-7867 and 9-7868, according to which a vitreous protective coating layer is formed on the surface of a rare earth/iron/boron permanent magnet by coating with an aqueous solution of an alkali silicate followed by a heat treatment for vitrification of the coating layer. This method in fact is a useful method at least when the surface-coated permanent magnet is employed in an atmosphere of air of which the humidity is not excessively high since the treatment method is relatively simple but still gives a considerably good rustproofing effect.
When a rare earth/iron/boron permanent magnet provided with a vitreous protective coating layer of alkali silicate is employed in an atmosphere of a relatively high humidity, on the other hand, the alkali constituent contained in the vitreous coating layer is responsible for absorption of moisture from the atmosphere. Once the coating layer is moistened by absorbing moisture, the desired effect of corrosion resistance can no longer be fully exhibited by the vitreous coating layer.
Moreover, the alkali constituent contained in the vitreous protective coating layer of alkali silicate is readily leached out into an aqueous or oily medium surrounding the magnet to cause heavy contamination around the magnet body. This problem, of course, can be at least partly solved by using an alkali silicate of which the content of the alkali constituent relative to the silica constituent is remarkably decreased. The amount of the alkali constituent relative to silica in the alkali silicate, however, cannot be low enough to be sufficient to avoid the trouble due to absorption of moisture by and leaching out of the alkali mentioned above because the alkali constituent in the alkali silicate acts to promote vitrification of the alkali silicate forming a coating layer in the heat treatment and to reduce shrinkage of the coating layer by vitrification so as to ensure good corrosion resistance of the vitreous protective coating layer.